Certain types of earth-moving and excavating machinery are equipped with digging buckets capable of holding anywhere from a fraction of a cubic yard to several cubic yards of material. One type of machine using a large digging bucket is called a walking dragline. Such draglines are often used in strip mining to remove "overburden" material covering, e.g., coal or ore, and to remove the product being mined. A large dragline may represent an investment of well over a million dollars; downtime is expensive, adds to the consumer cost of the product being mined and must be minimized.
Draglines are very large and include an enclosed machinery deck mounted on movable "legs" for machine transportability over a limited area. The machinery deck includes drive motors, cable reels, clutches and the like for manipulating a boom and boom-suspended bucket. The boom extends outward from the machinery deck by a distance of, for example, 300 feet or so. The digger bucket is attached to cables, one of which extends downward from the end of the boom to support the bucket weight and the other of which extends between the bucket and the machinery deck.
Digging is by lowering the bucket onto the material to be removed and dragging the bucket toward the machinery deck. As the bucket is drawn toward the machine, its digging teeth bite into the material as the bucket fills. After the bucket is filled, the boom is swung laterally and the bucket tipped for dumping the load. For a large dragline, the bucket capacity may be 80-90 cubic yards or even larger. And there are other types of machines, e.g., excavators, backhoes and the like, which use digger buckets mounted on articulated arms.
A large bucket may have several tooth assemblies, the individual components of which are typically quite large. For example, the tip of a digging tooth for a large bucket may have a length of 13 inches or so (as measured in the direction of digging), a width of about 12 inches and weigh about 160 pounds. And a large bucket itself may weight several thousand pounds.
More specifically, each hollow, sheath-like tip is fitted over and supported by a tooth "nose" or base. The tip (which is generally hollow to receive the base) has a pair of apertures, one each in the top and bottom tip plate. The base has a single vertical aperture. When the tip is fully seated on the base, the apertures are aligned and form a single vertical "top-to-bottom" aperture.
In conventional digging tooth assemblies, the tip is retained on the base by a wedge-shaped member sized so that when in place, a smaller aperture still remains. A wedge-shaped pin is driven (often with a sledge hammer) into this smaller aperture and retains the tip in place solely by friction. Because such digger buckets are subjected to severe use, often in hard mineral such as limestone, coal or rock, the bucket digging teeth wear and the above-described base/tip arrangement is configured in anticipation of periodic tip and/or wedge pin replacement.
The conventional wedge pin arrangement is attended by a number of disadvantages. One is that, over time, the wedge pins are knocked out or they can simply become loose and fall out. The tip then slips off of and falls from the base which may soon be broken off by continued digging. And repairing a broken base is a much more substantial task than replacing a worn tooth tip. Another disadvantage is that the wedge pin is usually hardened and repetitive hammering may cause the pin to splinter, sending shards of metal flying like shrapnel.
Yet another disadvantage is that the hardened wedge pin lacks significant resilience or ductility and this fact promotes pin loosening with slight wear. In other words, the parts are not self-adjusting to any significant degree.
Even if the wedge pin remains secure over the life of the tip, pin removal preparatory to tip replacement is a substantial task. Because such pins are driven from the top of the tooth downward, they must be removed by driving them upward using a hammer and drift pin. Access to the underside of the bucket is required to do this--and a bucket weighing several thousand pounds presents an imposing "positioning task." If the bucket teeth are merely lifted away from the ground (rather than totally inverting the bucket), an individual is required to work beneath the lifted bucket and this presents unnecessary risks.
Some wedge pin arrangements involve an aperture extending horizontally across the width of the tooth tip and tooth base. In theory, wedge pins should then be removable without gaining access to the bottom of the bucket. However, there is often too little space between tooth assemblies to permit either satisfactory wedge pin driving or later pin removal.
An improved apparatus and method for tooth tip retention which avoids drive pins, which eliminates a need for clear access to the bottom of the bucket and which retains the tooth tip by means other than merely friction would be an important advance in the art.